Electronic information handling or computer systems, whether large machines, microcomputers or small and simple digital processing devices, require memory for storing data and program instructions. Various memory systems have been developed over the years to address the evolving needs of information handling systems. One such memory system includes semiconductor memory devices.
Semiconductor memory devices are rapidly-accessible memory devices. In a semiconductor memory device, the time required for storing and retrieving information generally is independent of the physical location of the information within the memory device. Semiconductor memory devices typically store information in a large array of cells.
Computer, communication and industrial applications are driving the demand for memory devices in a variety of electronic systems. One important form of semiconductor memory device includes a non-volatile memory made up of floating-gate memory cells called flash memory. Flash memory is often used where regular access to the data stored in the memory device is desired, but where such data is seldom changed. Computer applications use flash memory to store BIOS firmware. Peripheral devices such as printers store fonts and forms on flash memory. Digital cellular and wireless applications consume large quantities of flash memory and are continually pushing for lower voltages and higher densities. Portable applications such as digital cameras, audio recorders, personal digital assistants (PDAs) and test equipment also use flash memory as a medium to store data.
Another important form of semiconductor memory device includes a volatile memory called dynamic random access memory (DRAM). DRAM is often used where rapid access to the memory array is desired for both data input and data output. DRAM has faster access times than flash memory, but requires periodic refresh to avoid losing its data values. Computer applications typically use DRAM to store program instructions and other temporary data.
Prior to shipping, a manufacturer may test its semiconductor memory devices as part of a quality program to improve end-use reliability. It is generally common to test the devices for defective columns or rows of memory cells, and to replace those defective elements with redundant elements. By writing a known pattern to a memory array, reading data values from the array and comparing those data values to the expected data values, defective elements can be identified. Such testing is generally performed by specialized external tester devices during the fabrication process and often prior to packaging the devices. Post-packaging failures are typically impracticable to detect as the specialized tester devices are generally unavailable.
For the reasons stated above, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative methods of testing memory devices, circuits for implementing such test methods, and memory devices making use of such circuits and test methods.